There are a few reasons I can think of.
-SD is cheaper for the same size/speed... A lot cheaper.
-SD is the more commonly used card for other devices like tablets, notebooks.

-I think partially as they expect some P&S'ers and lower model DSLRs to upgrade and most of them will have SD already.
-Smaller slot takes less room used in the camera.

-Some people may like that the SD is smaller so less room needed to carry a few cards.
Some people will love it and other not but I am still happy to get 2 slots.
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Michael Kaplan
http://www.pbase.com/mkaplan
See my profile for equipment list